Illumination devices (e.g., lamps) that comprise light emitting diodes (LEDs) as luminescent components usually cannot simply be connected to a voltage supply but have to be driven by special driver circuits (or control circuits) providing a defined load current to the LEDs in order to provide a desired radiant power (radiant flux). Since a single LED exhibits only small forward voltages (from about 1.5 V for infrared GaAs LEDs ranging up to 4 V for violet and ultraviolet InGaN LEDs) compared to commonly used supply voltages (for example, 12 V, 24 V and 42 V in automotive applications) several LEDs are connected in series to form so-called LED chains.
In many applications, it is desirable to have a fault detection included in the driver circuits (or control circuits) that allows for detecting defective LEDs in the LED chains connected to the driver circuit. An LED can be regarded as a two-terminal network. A defective LED can manifest in either an open circuit or a short circuit between the two terminals. If one LED of a LED chain fails as an open circuit this is relatively easy to detect because the defective LED interrupts the current for the whole LED chain. If one LED of a LED chain fails as a short circuit, however, only the defective LED stops radiating, which may be more difficult to detect.